An important design consideration in the development of a run-flat tire is insuring that the uninflated tire remains seated on the rim. Solutions have been developed employing bead restraining devices as well as special rims to accomplish this requirement.
The design of the bead region for extended mobility technology (EMT) tires requires the balance of two opposing criteria. The design of the bead region must provide the means for the bead to remain seated on the rim during uninflated operation and the design should be compatible with conventional wheel rims. The design should also allow the tire to be installed on a conventional wheel without special equipment or procedures and removed from a conventional wheel without damage to the tire. The prior art shows a variety of approaches, however, none provide a completely satisfactory solution to the problem or an optimum balance of operating characteristics when uninflated and compatibility with conventional wheels, tires, and mounting procedures.
The compressive force of the air in typical pneumatic tire is confined by the carcass plies in tension. The tensile force in the carcass plies is transmitted to the bead cores, placing the bead cores in tension. The bead core holds the bead seat radially inward against the rim of the wheel and in the case of tubeless tires maintains an air tight seal between the bead and rim bead seat of the wheel. The compressive force of the air in the tire also exerts an axial outward force on the bead region of the tire pressing the axially outward facing surfaces of bead seat against the rim flange. The relatively thin and flexible sidewalls of a normally inflated conventional tire do not transmit significant compressive forces, bending moments or axially inward forces to the bead region except possibly under conditions of severe cornering or breaking.
In comparison, EMT tires often rely on a thickened and reinforced sidewall construction to provide uninflated operation. These thickened and reinforced sidewalls are capable of transmitting compressive and bending stresses to the bead regions of the tire particularly during under-inflated or uninflated operation. These compressive and bending stresses conspire to unseat the bead from the rim particularly when combined with the additional lateral forces encountered in cornering. The compressive loading of the reinforced sidewalls carries an axial inward component tending to unseat the bead. The bending stresses tend to lift the radially and axially innermost edge of the bead or bead toe facilitating the slippage of the bead over any humps or protrusions that may be provided axially inward of the bead seat.
The prior art shows a considerable variety of approaches to the design a bead-wheel interface capable of resisting the axially inward forces that may be encountered during uninflated operation. Many of these approaches require modifications to the wheel rim as well as the bead region of the tire to provide and interlocking geometry where circumferential grooves or humps on the bead are matched by complementary grooves or humps on the flange and bead seat regions of the wheel. However the use of non-standards wheels and tires has not received wide application.
A more simple solution is to increase the size and strength of the bead core to improve the runflat operation. However, as the strength of the bead core is increased, mounting the tire on a wheel becomes more difficult and may require special equipment.
Another approach is to redesign the wheel and tire bead with a non-conventional geometry. For example, U.S. Pat. No. 5,145,536 ('536) discloses a bead profile with an elastomeric projection or toe extending radially and axially inward from the bead core and a wheel rim provided with a complementary groove. The wheel rim of the '536 patent is designed with an annular groove and hump that accommodate the toe and groove respectively of the tire bead.
U.S. Pat. No. 4,015,652 discloses a radial tire where at least one bead portion is adhered to the rim at one of the seats with a cement having sufficient bonding strength to retain the bead portion on the rim during operation of the tire in the deflated condition.
Despite the variety of ingenious approaches to improving the interface between the bead and the rim to provide more robust runflat operation, none of the existing designs have provided an entirely satisfactory solution.